1. Field of the Invention
The present invention relates, generally, to filtration aids, more particularly, a novel food grade polymer chemistry has been discovered for faster removal of gypsum from phosphoric acid/gypsum slurry to obtain higher phosphoric acid production rates.
2. Description of the Related Art
During the process of producing phosphoric acid (for fertilizer, animal feed, or food use), gypsum is formed as a by-product in the 25-45% phosphoric acid solution. This gypsum is filtered out with various types of vacuum filters. For additional phosphoric acid recovery, the gypsum is furthermore rinsed with fresh or recycled water several times in a reverse flow for maximum recovery. This rinse step adds two to three additional sections to the filter where the filtration rate can be improved. Many plants operate with filters at capacity making this the main limiting factor for phosphoric acid production. Furthermore the filtration rate is very dependent upon the type of phosphate rock added to the phosphoric acid reactor.
Polymeric filtration aids are often added to enhance filtration time for faster production and higher recovery of the phosphoric acid. U.S. Pat. No. 4,800,071 to Kaesler et al. and U.S. Pat. No. 5,211,928 to Rey et al., both discuss the use of various high molecular weight polymers for filtration aids used in gypsum slurry.
The normal application for powder polymers is by making a 0.5-2% polymer solution in water and injecting this into the gypsum/phosphoric acid slurry prior to filtration (by vacuum filter). The present invention also uses CMC made in a 0.5-2% polymer solution. The amount of polymer filtration aid for maximum performance varies with each production facility and phosphate rock source, as well as the polymer chemistry, and ranges from 2-150 ppm (via volume of slurry). The amount of performance improvement (over untreated slurry) by the filtration aid varies from 5-70%, and is typically<15% for igneous rock and also<20% for Florida sedimentary rock.